Instrumentation for determining loading and speed characteristics of motors is well known. Such instrumentation interpreting signals obtained from motor speed and analyzing slip characteristics of induction motors for example is described in my U.S. Pat. Nos. as follows:
4,063,112 for Induction Motor Load Monitor and Control Apparatus, Dec. 13, 1977.
4,101,831 for Load Monitoring Apparatus and Method, July 18, 1978.
4,141,244 for Load Indicating Metering Devices for Motor Driven Loads, Feb. 27, 1979.
4,194,129 for Armature Slip Analysis of Induction Motors, etc., Mar. 18, 1980.
4,194,178 for Electric Motor with Internal Wireless Load Monitor, Mar. 18, 1980.
A deficiency of this prior art, however, that limits its applicability to retrofit of both existing and new installations because of inconvenience or cost is the requirement for modification of the motor or system in such a way that signals can be derived representative of motor rotation data from which other motor characteristics are derivable.
Thus, it is an objective of the present invention to provide improved motor characteristic sensing means compatible with this prior art technology that permits immediate installation or use of instrumentation at a motor site without any access to the motor by means of mechanical, photo, or electrical modification or precise focussing, alignment or positioning of sensitive probes such as photo electric cells or those which must be precisely spaced and thus which cannot be used in limited access existing motor installations. With such improvement in motor characteristic sensing means the foregoing instrumentation and other instrumentation such as motor speed indicators can be made feasible for use in almost any existing motor installation.
Some prior art attempts have been made to sense motor characteristics without motor modifications. However, such motor characteristic sensing devices are very limited in application and function.
Thus, the sensing technique described in U.S. Pat. No. 4,138,642--Feb. 6, 1979 provides a sensing coil external to the motor, oriented with an axis perpendicular to the rotor of a permanent magnet motor at a critical position near a leading end of the permanent magnet, to derive variations in stray flux for providing an r.p.m. pulse count. This technique is limited to permanent magnet d-c motors and cannot be applied to a-c induction motors for example. Also, it is inoperable if the sensor coil is misplaced and thus cannot be used if the motor installation limits accessibility.
There are significant problems involved in sensing a-c motor stray flux because of the dominant strength of the basic generally 60 Herz power line frequency component as a result of high current flow therefrom. The stray flux of the much smaller magnitude rotational fields has thus prevented use of techniques of this type heretofore in the art, even though they are useful in the d-c permanent magnet motor art where no dominant a-c line frequency field exists, although subject to critical placement of the sensors both circumferentially and axially. Thus, the present invention has as an objective the provision of a sensor operable within the high line current field effect of an a-c induction motor.
In addition, it is an objective to provide an uncritical sensor position that is therefore operable at uncritical accessible locations on already existing motor installations in limited space, etc., and which are not subject to critical positioning by a skilled technician for operability.
Another sensor of limited application for determining the direction of rotation of an a-c electric motor is described in U.S. Pat. No. 3,943,504--Mar. 9, 1976. However, it cannot be used in the presence of the a-c current in an operating motor and must have two pick-up locations critically spaced at an angle related to the rotating field characteristics of the motor being tested. A separate high frequency source of test voltage is also required.
U.S. Pat. No. 4,179,744-Dec. 18, 1979 describes a method of analyzing the magnetic fields of passing d-c railway traction motors by pick-up coils located on the railroad ties to measure the passing of the opposite poles of the motor with the engine. This equipment has a limited purpose of sensing which motors are in proper running condition at full r.p.m. by sensing a limited number of cycles of rotation over train travel along the length of the motor. Also the alternator frequency can be determined.
There has been no known prior art instrumentation for enabling sensing equipment to isolate and process rotational signals from dominating line current noise in a-c induction type motors and no known solutions have been provided heretofore for the hereinbefore mentioned problems and deficiencies.
Other objects, features and advantages of the invention will be found throughout the following description, the drawings and the claims.